Stabilized electroless copper solution



United States Patent Oflice 3,222,195 Patented Dec. 7, 1965 3,222,195 STABILIZED ELECTROLESS COPPER SOLUTION Fred Pearlstein, Philadelphia, Pa., assignor to the United States of America as represented by the Secretary of the Army 'No Drawing. Filed Feb. 23, 1962, Ser. No. 175,372 8 Claims. (Cl. 106-1) "(Granted under Title 35, US. Code (1952), sec. 266) The invention described herein may be manufactured and used by or for the Government for governmental purposes without the payment to me of any royalty thereon.

This invention relates to electroless copper solutions and more particularly 'concerns such solutions having improved resistance to general and rapid decomposition.

Electroless copper solutions, in the main, are Fehlings solutions or variations thereof, having incorporated therein a reducing agent such as formaldehyde. Such solutions generally start decomposing within about 20 minutes after make-up and cannot ordinarily be used for deposition afterabout 3 hours.

Electroless copper solutions reported in the literature, comprise Fehlings solutionand formaldehyde with substantial quantities of complexing agents added thereto, such as sodium carbonate, Versene, andthe like. The stability of these solutions, and the rate at which deposits form therefrom leaves much to be desired.

Accordingly, it is an object of this invention to provide a highly stable electroless copper solution.

Another object of the invention is to provide such a solution as aforementioned which requires a very minor amount of an abundantly available compound.

Other and further objects of the invention will be apparent to those skilled in the art upon study of this disclosure.

Briefly, I have found that minor additions of 2-mercaptobenzothiazole (MBT) to diluted Fehlings solution and formaldehyde produces a marked increase in the stability of the resultant electroless copper solution. More specifically, when 2-mercaptobenzothiazole is added in amounts ranging between about 0.0005 to 0.008% Weight/ volume of the original Fehlings solution (the expression percent weight/volume being defined as grams of solute/ 100 milliliters of solvent and hence 0.0005 to 0.008% weight/volume equals 5 to 80 milligrams/liter) and to which is added formaldehyde, the resultant solution permits good deposition therefrom at rates faster than any of the known proprietary electroless copper solutions and yet its stability is vastly improved.

In accordance with my invention, Fehlings solution, as specified by the Association of Official Agricultural Chemists, consists of 34.64 g./l. CuSO .5H O, 173 g./l. rochelle salts.4H O, 50 g./l. sodium hydroxide and the balance water. The 2-mercaptobenzothiazole is readily added to the Fehlings solution from a stock solution consisting of '10 g./l. Z-MBT dissolved in .2 N sodium hydroxide solution. 100 ml. of 36% formaldehyde (containing 12 /2% methanol as a preservative) will then be added to the 2- mercaptobenzothiazole-Fehlings solution for each liter of original Fehlings solution present. The results of deposition tests conducted at a solution temperature of 25 C. and with varying solutions are shown below in Table I. Each solution was allowed to stand for at least 1 hour before running the tests.

Table I [Electroless copper deposition] The weight per volume percentage of 2-mercaptobenzothiazole given in Table I above and Table II to follow is'bas ed on the undiluted Fehlings solution.

Distilled water and reagent grade chemicals werc used in preparing the solutions describedherein. ,The electroless copper solution without 2-mercaptobenzo'thiazole coinprisesFehlings'solution and ml. of'36% formaldehyde for each liter of Fehlings solution. At the 50% concentration without 2-mercaptobenzothiazole, the Fehlings solution is diluted 1 to l with distilled water and 100 ml. of 36% formaldehyde added per liter of undiluted Fehlings solution. The 2-mercaptobenzothiazole may be added directly to the Fehlings solution or the Fehlings solution may be diluted and the Z-mercaptobenzothiazole added thereto. In any case, the Z-mercaptobenzothiazole will be added prior to the formaldehyde.

Poor bath stability is defined as one in which the bath is substantially decomposed within about 3 hours; a fair bath stability being one which is relatively undecomposed within about 3 hours but almost completely decomposed within 24 hours; and a good bath stability showing little or no decomposition after about 24 hours.

A poor deposit appearance is discontinuous or powdery whereas a good deposit appearance is coppery metallic. The appearance of the fair deposit lies therebetween.

Deposition tests were performed on a 2-in. x 2-in. cleaned and sensitized Mylar sheet of 0.003 inch thickness.

It is apparent from the data presented in Table I that dilution, within the limits shown, improves bath stability but decreases deposition rates. To achieve a good workable compromise between deposition rates and stability, I prefer an electroless copper soltuion consisting of 40% Fehlings solution with 0.0032% weight/volume of 2- mercaptobenzothiazole which is based on the undiluted Fehlings solution. To this is added 100 ml. of the aforedescribed formaldehyde solution for each liter of the undiluted Fehlings solution present, the resultant solution being used between the range of about 20 to 30 C. In Table 11 below, comparison data are presented on my stabilized electroless copper solution and three widely used proprietary solutions. The tests were carried outwith a bath temperature of about 27 C. using the 40% solution aforedescribed.

1 Solution completely decomposed.

2 Poor, discontinuous deposit.

Brand X, Y and Z are modified Felilings solution-formaldehyde solutions.

It is apparent from the foregoing tables and description that I have provided an eleetroless copper solution which is more stable than each of the three well-known and widely used commercial solutions. The appearance of my deposit after my solution had been made up for 192 hours was still good and copper was deposited at the rate of about 14 milligrams in 15 minutes. Further, the 2-mercaptobenzothiazole is incorporated in the Fehlings solution in very minor amounts only and is abundantly available. The principal advantage of my inventive electroless copper solution however resides in its ability to be workable for at least several days thus obviating the necessity for many time consuming solution make-ups.

I claim:

1. In an electroless copper plating solution consisting essentially of Fehlings solution and a reducing agent, the combination therewith of a stabilizing amount of 2-mercaptobenzothiazole.

2. In an electroless copper plating solution consisting essentially of Fehlings solution and formaldehyde, the combination therewith of a stabilizing amount of Z-mercaptobenzothiazole.

3. In an electroless copper plating solution consisting essentially of Fehlings solution and aqueous formaldehyde solution in the volume ratio of 10 to 1, respectively, the combination therewith of a stabilizing compound consisting of about 5 to 80 milligrams of Z-mercaptobenzothiazole per liter of undiluted Fehlings solution.

4. An electroless copper plating solution according to claim 3, said solution comprising an additional volume of water ranging from 1 to 3 times the volume of undiluted Fehlings solution.

5. A process for improving stability of an electroless copper plating solution consisting essentially of Fehlings solution and a reducing agent, comprising adding a stablizing amount of Z-mercaptobenzothiazole to said electroless copper solution.

6. A process according to claim 5 wherein said reducing agent consists of formaldehyde.

7. A process for improving stability of an electroless copper plating solution consisting essentially of Fehlings solution and aqueous formaldehyde solution in the volume ratio of 10 to 1, respectively, comprising adding to said electroless copper solution 5 to 80 milligrams of 2-mercaptobenzothiazole per liter of undiluted Fehlings solution.

8. A process according to claim 7 including the step of adding to said electroless copper solution an additional volume of water ranging from 1 to 3 times the volume of undiluted Fehlings solution.

References Cited by the Examiner UNITED STATES PATENTS 2,429,970 10/1947 Wernlund 20446 2,663,684 12/1953 Pierce 20452 3,033,703 5/1962 Schneble 1061 ALEXANDER H. BRODMERKEL, Primary Examiner.

JOHN R. SPECK, Examiner. 

1. IN AN ELECTROLESS COPPER PLATING SOLUTION CONSISTING ESSENTIALLY OF FEHLING''S SOLUTION AND A REDUCING AGENT, THE COMBINATION THEREWITH OF A STABILIZING AMOUNT OF 2-MERCAPTOBENZOTHIAZOLE. 